The present invention relates to instrumentation and methods for measuring the amount of use of a joint in a skeletal system. The invention is useful as applied to natural joints and artificial implants. The invention may be applied to a wide variety and types of implants. The invention may have particularly useful application to joint prostheses including hip, knee, shoulder, ankle, wrist, jaw, and spinal prostheses. Joint prostheses are usually manufactured of durable materials such as metals, ceramics, or hard plastics and are affixed to articulating ends of the bones of the joint. Joint prostheses usually include an articulating surface composed of a material designed to minimize the friction between the components of the joint prostheses. For example, in a hip prosthesis the femoral component is comprised of a head (or ball) and a stem attached to the femur. The acetabular component is comprised of a cup (or socket) attached to the acetabulum and most often includes a polyethylene articulating surface. The ball-in-socket motion between the femoral head and the acetabular cup simulates the natural motion of the hip joint and the polyethylene surface helps to minimize friction during articulation of the ball and socket.
It has been shown that over time implants will begin to degrade due to normal wear of the implant. Extensive or excessive use of the implant further increases the likelihood of implant degradation. Implant degradation, in turn, may cause polyethylene wear debris and implant loosing that can result in numerous medical problems including, but not limited to, osteolytic lesions and the possibility of requiring revision surgery. Early detection of the signs of implant degradation could allow an orthopedic surgeon to treat the potential problem before it escalates to the point of causing harm to the patient or the need for revision surgery.
Patients that extensively use or place repetitive loads on their implants are more likely to have implant degradation. Therefore, for more active patients it is necessary to carefully monitor the degradation of the implant with frequent examinations. However, these examinations are usually expensive, inconvenient, and often add undesired x-ray exposure to the patient. Thus, it is desirable to perform these examinations only when the patient's activity and implant usage levels are sufficiently high so as to increase the likelihood of implant degradation.
Therefore, there remains a need for improved instrumentation and methods for measuring the amount of use of a joint.